ci: nodejs in job image, fetch-depth=0 for base diff

.gitea/workflows/validate-pr.yml

@@ -10,6 +10,8 @@ jobs:
     runs-on: self-hosted
     steps:
       - uses: actions/checkout@v4
+        with:
+          fetch-depth: 0           # need full history for `git diff base...HEAD`
 
       # 1. Identify which recipes the PR touches.
       - name: detect changed packages

runner/flake.nix

@@ -42,6 +42,7 @@
             curl
             jq
             tea
+            nodejs_20            # JS-based actions/checkout@v4 etc. need `node` on PATH
 
             cacert
             iana-etc